Electricity is a primary power source used in residential and commercial applications. Electricity is generally transmitted as alternating current. Electricity can be supplied as single-phase or single-phase/split-phase or poly-phase (i.e. multiphase). In single-phase or poly-phase electricity distribution systems, each phase supplied generally requires a separate power line. However, a single-phase/split-phase electricity distribution system typically includes a three-wire configuration on the load side of a distribution transformer. The three wires include two live conducting wires and a mid-point neutral wire (i.e. earth ground). The single-phase/split-phase electricity distribution system is commonly used in North America for single-family residential and light commercial applications.
Measuring the amount of electricity drawn is useful for determining how much to charge a user and for planning future capacity requirements. Electricity is generally measured in terms of an amount of total energy consumed. Total energy consumption generally involves the integration of power used over a specified period of time. Power (P) is the product of voltage (V) and current (I). Electricity meters generally measure total power consumed by measuring the product of the voltage and the current at a given instant in time and accumulating (e.g. integrating) that product over time.
One way to measure current is by using a current shunt. A current shunt generally includes a relatively small-value resistor which is placed in series with a load. Current flows through the current shunt creating a voltage drop across the shunt. A voltage meter or electronic device capable of acquiring the voltage is used to measure the voltage drop, which is used to calculate the current. However, conventional current shunts are not preferred for single-phase/split-phase or poly-phase power measurement, or generally where two or more shunt current sensors are required, for a number of reasons. Current shunts are not isolated from power lines. Consequently, in power metering, sensed phase-to-phase or phase-to-neutral voltage differences produced on the input pins of a metering device can easily exceed maximum levels allowed in standard semiconductor products. Conventional current shunts also need to be electromagnetically isolated from one another to inhibit electromagnetic coupling that sometimes prevents accurate single-phase/split-phase or poly-phase current measurements due to magnetic crosstalk between shunt sensor elements. Achieving substantial magnetic isolation typically requires surrounding each current shunt with a shielding material. But the shielding material makes metering equipment heavy and more expensive. The shielding material can also cause undesirable phase shifts between current and voltage on each shunt that requires compensation and reduces measurement accuracy. Thus, while current shunts are often less expensive, current transformers and Rogowski coils are preferred for commercial single-phase/split-phase and poly-phase metering applications.